The present invention relates to a wireless communication apparatus which transmits or receives a video signal, a sound signal-and the like on a radio line, and more particularly, a wireless communication apparatus which ensures synchronicity of a signal to be transmitted between the sending side and the receiving side via an asynchronous radio transmission line.
The recent years have seen an increasingly popular use of a wireless local area network (hereinafter referred to as an “LAN”), which typically is IEEE802.11, instead of a conventional wired LAN such as the Ethernet, etc., owing to advantages of a wireless LAN such as the higher degree of freedom regarding the installation locations of equipment. Meanwhile, a wireless communication apparatus which transmits or receives a video signal, a sound signal and the like using a wireless LAN is becoming popular. A conventional wireless communication apparatus will now be described below.
FIG. 12 is a block diagram showing the structure of a conventional wireless communication apparatus. In FIG. 12, denoted at 10A is a first wireless communication unit which encodes and modulates a received video signal and sends the video signal as an electric wave having a predetermined format, and denoted at 10B is a second wireless communication unit which receives, demodulates and decodes the electric wave and outputs a video signal.
The first wireless communication unit 10A comprises a video signal encoding part 400A, a communication protocol processing part 600A, a MAC (Media Access Control) processing part 700A, an RF part 800A, and an antenna 30A, and is connected with the second wireless communication unit 10B via a radio line.
The second wireless communication unit 10B comprises a video signal decoding part 500B, a communication protocol processing part 600B, a MAC processing part 700B, an RF part 800B, and an antenna 30B.
Although the first wireless communication unit 10A and the second wireless communication unit 10B are both capable of performing bidirectional communications, for the simplicity of illustration, FIG. 12 shows a transmission path in only one direction.
FIG. 13 is a timing chart showing the timing of signals which are sent onto the radio line.
FIG. 14 is a block diagram showing the internal structures of the MAC processing part 700A of the first wireless communication unit 10A and the MAC processing part 700B of the second wireless communication unit 10B.
In the section (A) in FIG. 14, denoted at 704A is a frame assembling part, denoted at 701A is a clock generating part, denoted at 702A is a beacon timer part, denoted at 703A is a beacon generating part, denoted at 705A is a frame analysis part, and denoted at 706A is a receive data processing part.
In the section (B) in FIG. 14, denoted at 704B is a frame assembling part, denoted at 701B is a clock generating part, denoted at 702B is a beacon timer part, denoted at 703B is a beacon generating part, denoted at 705B is a frame analyzer, and denoted at 706B is a receive data processing part.
Operations of the conventional wireless communication apparatus having such a structure as above will now be described.
First, operations of the first wireless communication unit 10A will be described. A video signal is fed to the first wireless communication unit 10A from outside. The video signal encoding part 400A compresses the inputted video signal into a predetermined format and accordingly encodes the video signal. For transmission of a video signal, the video signal is encoded using the MPEG2-TS format in general for the purpose of reducing the amount of transmission data, transmission of clock information, etc. Thus encoded video signal is assembled into a packet in the MPEG2-TS format.
The MPEG2-TS packet is supplied to the communication protocol processing part 600A, whereby the MPEG2-TS packet is further assembled into a packet based on a predetermined communication protocol and a necessary header is added to this packet. TCP/IP for realizing reliable data transmission, UDP/IP which is suitable to transmission of stream data or the like may be used as this communication protocol, so that the MPEG2-TS packet is assembled into a packet of the IP packet format.
The IP packet is fed to the MAC processing part 700A, to be thereby assembled into a MAC packet based on a predetermined wireless communication scheme. As the structure of the MAC packet, the scheme defined by wireless LAN standards such as IEEE802.11, etc. is used.
The data assembled into the MAC packet format are supplied to the RF part 800A. The RF part 800A executes predetermined modulation and sends out via the antenna 30A the data as an electric wave which has a predetermined frequency. As the modulation method, quadrature modulation and spectrum diffusion or OFDM, etc. are used. The frequency may be the 2.4G band, 5G band, etc. Thus inputted video signal is sent to the radio communication line by a predetermined method.
Operations of the second wireless communication unit 10B shown in FIG. 12 will be now described. In the second wireless communication unit 10B, the electric wave (data) received at the antenna 30B is supplied to the RF part 800B.
The RF part 800B selects a desired frequency, demodulates and converts the receive data into a baseband signal, namely, data of the MAC packet format, and outputs the data to the MAC processing part 700B.
The MAC processing part 700B analyzes thus inputted data of the MAC packet format, converts the data into an IP packet and outputs the IP packet to the communication protocol processing part 600B.
The communication protocol processing part 600B extracts an MPEG2-TS packet from the received IP packet and outputs the MPEG2-TS packet to the video signal decoding part 500B.
The video signal decoding part 500B decodes the received MPEG2-TS packet, decompresses the packet and outputs the packet as a video signal. In this manner, the video signal received from the first wireless communication unit 10A is transmitted to the second wireless communication unit 10B via the radio communication line, decoded and decompressed in the second wireless communication unit 10B, and outputted to outside.
The timing of signals which are sent onto the radio line will now be describe with reference to FIG. 13. In FIG. 13, a beacon is a control signal from the first wireless communication unit 10A, and is transmitted in constant intervals (cycles) of from about a few milliseconds to 1 second. The beacons contain control information such as an identification number for identifying the first wireless communication unit 10A and the count of a timer which is disposed inside the first wireless communication unit 10A.
The beacons are used for the purpose of allowing the second wireless communication unit 10B to identify the first wireless communication unit 10A based on the identification number which is contained in the beacons, for the purpose of adjusting the timing of sending and receiving on the part of the first wireless communication unit 10A and the second wireless communication unit 10B based on the timing at which the beacons are outputted, for the purpose of matching an internal timer of the second wireless communication unit 10B with the timer count of the first wireless communication unit 10A contained in the beacons so that the second wireless communication unit 10B intermittently performs a receiving operation only at beacon receiving timing thereby saving electric power when it is not necessary to transfer video data between the first wireless communication unit 10A and the second wireless communication unit 10B, and for other purposes.
Video data are sent out at timing between two neighboring beacons. IEEE802.11 uses the CSMA/CA scheme. Control according to this scheme requires, for sending of data, to receive data first, confirm whether a radio line is unoccupied, i.e., whether other wireless communication apparatus is sending data to the radio line, and send data if the radio line is unoccupied but confirm that sending from the other wireless communication apparatus ended and thereafter send the data.
Hence, in the event that there are a plurality of second wireless communication units 10B, when the radio line is being used for other communication, sending has to wait until sending of the signal for the other communication completes and the radio line becomes available. Depending on the state of use of the radio line, this sometimes delays transmission.
With reference to FIG. 14, explanation will now be given on an operation in which the first wireless communication unit 10A sends the beacons, the second wireless communication unit 10B receives the beacons and the internal timer of the first wireless communication unit 10A and the internal timer of the second wireless communication. unit 10B are synchronized to each other.
Sending and receiving of the beacons is performed by the MAC processing part 700A of the first wireless communication unit 10A and the MAC processing part 700B of the second wireless communication unit 10B.
First, an operation that the first wireless communication unit 10A sends the beacons will be described. The clock generating part 701A generates a clock signal which is for creating the timing at which the beacons are to be sent out, and outputs the clock signal to the beacon timer part 702A.
The beacon timer part 702A counts the clock signal. The beacon timer part 702A is comprised of a counter having approximately 64 bits for instance or the like.
Referring to the count (timer count) outputted from the beacon timer part 702A and upon detection that the timer count changed to a predetermined value, the beacon generating part 703A outputs to the frame assembling part 704A control information which is contained in the beacons, namely, the identification number for identifying the first wireless communication unit 10A and the timer count outputted from the beacon timer part 702A. The predetermined value is a value which is determined by a relationship with the clock signal of the clock generating part 701A, and desired beacon sending cycle is set in accordance with this value. The frame assembling part 704A stores the control information in a predetermined MAC frame and outputs the same to the RF part 800A. In this manner, a beacon signal in which the control information is stored in a predetermined cycle is sent out.
An operation that the second wireless communication unit 10B receives the beacons will now be described.
The frame content of the received signal outputted from the RF part 800B is analyzed by the frame analysis 705B in the second wireless communication unit 10B, whereby whether this signal is addressed to the second wireless communication unit 10B is judged. This signal is discarded when this signal is not addressed to the second wireless communication unit 10B, whereas when this signal is addressed to the second wireless communication unit 10B, the receive data are outputted to the receive data processing part 706B.
The receive data processing part 706B, when the receive data are the beacons, extracts the control information which is contained in the beacons, extracts information regarding the timer count from the control information, and outputs the information to the beacon timer part 702B. This timer count is the count of the timer of the first wireless communication unit 10A stored in the beacons by the first wireless communication unit 10A.
The beacon timer part 702B of the second wireless communication unit 10B counts the clock signal outputted from the clock generating part 701B, similarly to the beacon timer part 702A of the first wireless communication unit 10A. A difference from the first wireless communication unit 10A is that the count of the beacon timer part 702B of the second wireless communication unit 10B is rewritten by the timer count which is outputted from the receive data processing part 706B. The rewriting matches the count of the beacon timer part 702A of the first wireless communication unit 10A with the count of the beacon timer part 702B of the second wireless communication unit 10B.
For every receipt of the beacon, the count of the beacon timer part 702B of the second wireless communication unit 10B is rewritten by the timer count of the first wireless communication unit 10A contained in the received beacon, whereby the internal timer of the first wireless communication unit 10A and the internal timer of the second wireless communication unit 10B are synchronized to each other.
Since the internal timer of the first wireless communication unit 10A and the internal timer of the second wireless communication unit 10B are synchronized to each other, operations of the second wireless communication unit 10B can be controlled in such a manner that the second wireless communication unit 10B performs a receiving operation only at the timing that the first wireless communication unit 10A sends out the beacons but otherwise stops performing this operation, which reduces consumption power.
The conventional wireless communication apparatus has such a structure as described above and is convenient in that it is possible to supply video signals from the first wireless communication unit 10A to the second wireless communication unit 10B without installing a new wired transmission line. However, due to an influence of a change in communication state and propagation delay, delay in communication protocol processing and the like which are unique to a radio line, there are some cases that a video signal reproduced in the second wireless communication unit 10B cannot be reproduced normally.
While video signals are transmitted in the MPEG2-TS format, during transmission of MPEG2-TS packets via a radio line, but for accurate reproduction of the timing at which the video signal encoding part 400A outputs an MPEG2-TS packet and the timing at which the video signal decoding part 500B receives an MPEG2-TS packet, a video signal cannot. be reproduced normally on the receiving side.
This is because an MPEG2-TS packet contains clock information which serves as a reference clock for decoding and therefore a correct reference clock can not be reproduced unless transmitted at the correct timing.
However, in the case of the wireless communication apparatus having such a structure, a processing delay time at the communication protocol processing part sometimes becomes short or long.
Although the processing performed in the communication protocol processing part requires an extremely complex configuration if to be realized by hardware, and therefore is realized by software using a CPU and the like, it is difficult to ensure that a processing time has a constant value in processing realized by software.
In addition, when a radio communication line is used, since a propagation path does not remain constant but changes because of reflection of an electric wave by a surrounding structure or for other reasons, a processing delay time changes.
Further, when the quality deteriorates due to jamming or interference from other wireless communication apparatus, the MAC processing part executes complementary processing such as to detect an error in data to be transmitted and to resend the data. In this case, since data transmitted once again are re-sent later, a long delay time is created.
Hence, in this case, there is a problem that the output timing of an MPEG2-TS packet from the video signal encoding part 400A fails to be accurately reproduced upon arrival at the video signal decoding part 500B so that a video signal cannot be reproduced normally on the receiving side.